What would be the most efficient way to retopo this mesh?
The geometry looks OK. You do not need to retopologize this. It can be improved by editing and removing some loops or sliding some edges(Shift + V) and merging some vertices, but I don't think you need to do a lot here. Starting from scratch seems unnecessary for many purposes. Obviously, since you did not tell anything about your context or what you are planing to use it for, it's hard to say for sure.